Former US President Bill Clinton has admitted his "three strikes" crime bill introduced in the 1990s contributed to the problem of overpopulated prisons.

Speaking to a civil rights group, he said: "I signed a bill that made the problem worse and I want to admit it."

It put 100,000 more police officers on the streets but locked up "minor actors for way too long", Mr Clinton said.

President Barack Obama launched a renewed effort to reform the criminal justice system this week.

He visited a federal prison in Oklahoma on Thursday, becoming the first sitting president to do so.

Speaking at the El Reno Federal Correctional Institution, Mr Obama said the criminal justice system needs to distinguish between young people who make mistakes and those who are truly dangerous.

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Some of the young prisoners he met at the prison had made mistakes not that different from those he made in his youth, Mr Obama said.

On Wednesday, Mr Clinton told the National Association for the Advancement of Coloured People (NAACP) convention in Philadelphia that he had faced a "roaring decade of rising crime".

He signed a bill into action in 1994 that included a "three strikes" rule that meant anyone convicted of a serious violent crime who had two or more prior convictions, including drug crimes, was given a life sentence.

He said the "good news" was that the tough raft of measures in the bill had helped secure "the biggest drop in crime history".

"The bad news is we had a lot of people who were essentially locked up who were minor actors for way too long," he added.